In 2005, as a measure to deal with global warming, the Kyoto protocol regarding reduction of carbon dioxide emission is enacted. Accordingly, there is an imminent need for providing methods for efficient use of carbon dioxide. In addition, as Clean Development Mechanism (CDM) industries are developed actively, particularly in advanced countries, carbon emission trading may be allowed according to the reduction of carbon dioxide. Therefore, it is expected that a success in development of efficient carbon dioxide utilization technologies has a great ripple effect in terms of economy. As processes for converting carbon dioxide, there have been suggested processes for preparing synthesis gas from combined reforming of natural gas, carbon dioxide and steam, and then producing useful chemical materials and transportation fuel from synthesis gas. Particularly, a process for synthesizing methanol, one of the most important chemical materials, or a process for preparing synthetic oil via Fischer-Tropsch synthesis by using synthesis gas obtained from the combined reforming, is getting important because it is regarded as an efficient way for utilization of carbon dioxide.
Processes for preparing synthesis gas from natural gas may be classified broadly into steam reforming of methane (SRM), partial oxidation of methane (POM) with oxygen, and carbon dioxide reforming of methane (CDR). The ratio of hydrogen to carbon monoxide (H2/CO) is varied depending on the reforming process and should be adjusted to the optimal condition required for the subsequent process. In the case of a highly endothermic SRM process, it is possible to obtain a ratio of H2/CO of 3 or higher. Thus, the process is suitable for hydrogen production and ammonia preparation. In the case of POM, a ratio of H2/CO of about 2 is obtained. Thus, it is known that POM is suitable for methanol synthesis reaction and hydrocarbon formation through Fischer-Tropsch synthesis. However, POM is disadvantageous in that it requires a large-scale air-separation unit (ASU).
Hereinafter, the above-mentioned reforming processes are outlined with their advantages, disadvantages and the values of heat of reactions.
Steam Reforming of Methane (SRM)
CH4+H2O=3H2+CO ΔH=226 kJ/mol
→ highly endothermic reaction, H2/CO>3, excess steam is required.
Partial Oxidation of Methane (POM)
CH4+0.5O2=2H2+CO ΔH =−44 kJ/mol
→ mild exothermic reaction, H2/CO=2, O2 production process is required.
Carbon Dioxide Reforming of Methane (CDR)
CH4+CO2=2H2+2CO ΔH=261 kJ/mol
→ highly endothermic reaction, H2/CO=1, CO2 addition is required.
In addition to the above reforming processes, there are known an auto-thermal reforming (ATR) process which is a combination of POM and SRM, a tri-reforming process which is a combination of POM, SRM and CDR, or the like in order to make an adequate H2/CO ratio as well as to increase energy and carbon efficiency. Further, it is possible to obtain synthesis gas having different H2/CO ratios depending on the type of reforming process and catalyst. Recently, many patent applications related to different methods using synthesis gas with such different H2/CO ratios (Korean Unexamined Patent Publication Nos. 2006-0132293 and 2005-0051820).
According to the present disclosure, a nickel-based reforming catalyst (Ni/Ce(Zr)MgAlOx) is used to carry out steam carbon dioxide reforming of methane (SCRM), wherein the catalyst has high catalytic activity in the combined reforming in which SRM is carried out simultaneously with CDR, as disclosed in Korean Patent Application No. 2008-0075787 derived from our previous study. In this manner, synthesis gas is prepared to maintain carbon monoxide, carbon dioxide and hydrogen at a suitable composition [H2/(2CO+3CO2)] and a catalyst for combined reforming, which is suitable for methanol synthesis and Fischer-Tropsch synthesis using iron-based catalysts, is used. It is shown that the catalyst is inhibited not only from deactivation caused by carbon generation during the reaction but also from deactivation caused by nickel reoxidation due to water added during the reaction. Thus, the catalyst has excellent reactivity as compared to other known catalysts for combined reforming processes. In general, it is known that a ratio of H2/(2CO+3CO2) in synthesis gas of about 1.05 is thermodynamically suitable for methanol synthesis. By adjusting the ratio to an adequate range, it is possible to increase methanol yield and carbon utilization efficiency. Therefore, it is required to add hydrogen in order to adjust the above ratio, or to modify processing parameters (temperature, pressure, etc.) in order to adjust the CO2 conversion in CDR.
In the case of a currently available SRM process, a Ni/Al2O3 catalyst system is used at a reaction temperature of 750 to 850° C. under a molar ratio of steam/methane of 4-6:1. However, such a catalyst system is problematic in that it undergoes severe deactivation caused by carbon deposition. Therefore, many studies have been conducted about catalyst systems containing noble metals or transition metals and alkali metals as co-catalysts (Journal of Molecular Catalysis A 147 (1999) 41). In addition, in the case of a CDR process, more severe deactivation of catalysts occurs due to carbon deposition. Therefore, in order to inhibit such catalyst deactivation, many studies have been conducted about noble metal catalysts (Pt/ZrO2) and Ni/MgO or Ni/MgAlOx catalyst systems, to which alkali metals are added as co-catalysts (Catalysis Today 46 (1998) 203, Catalysis Communications 2 (2001) 49, and Korean Unexamined Patent Publication No. 10-2007-0043201). In general, when using commercially available SRM catalysts directly to CDR and combined CDR and SRM processes, deactivation of catalysts caused by carbon deposition is accelerated.
It is generally known that methanol is produced from synthesis gas via the hydrogenation of carbon monoxide or carbon dioxide as depicted in the following reaction formulae:CO+2H2 CH3OH ΔH=−90.8 kJ/mol  (4)CO2+3H2 CH3OH+H2O ΔH=−49.6 kJ/mol  (5)CO+H2O CO2+H2 ΔH=−41.2 kJ/mol  (6)
Reaction formulae (4) and (5) are kinds of the exothermic volume-reducing reactions, and thus they prefer a low temperature and a high pressure thermodynamically. However, commercial production of methanol has been conducted at an adequate temperature to increase the reaction rate. In addition, the unreacted gases are recycled and used again in methanol synthesis in order to increase the availability of synthetic gas and to improve the conversion into methanol. However, water produced according to Reaction Formula (5) causes water gas shift reaction (WGS), such as one as shown in reaction formula (6), thereby forming an excessive amount of CO2 as a byproduct. Therefore, when introducing a catalyst and process capable of improving the yield of methanol synthesis by adjusting such parameters adequately, it is possible to improve the carbon utilization and energy utilization efficiency of the overall process. In this context, many workers have participated in studies for improving the quality of catalysts for methanol synthesis, but complete understanding about the active site of a catalyst for methanol synthesis cannot be accomplished heretofore. However, it is known that oxidation state of Cu and redox conversion property of reduced Cu particles play an important role in determining the catalyst quality. It is also known that the activity of a Cu catalyst in a reaction of methanol synthesis is in proportion to the specific surface area of Cu of the metal components. For this reason, Cu is used frequently in combination with Zn to prepare the catalyst, and a molar ratio of Cu/Zn of 3/7 is known to provide the highest activity. However, it is known that when CO2 is present or when the proportion of oxygen-containing materials that cover the Cu0 surface increases, the catalyst activity is independent from the Cu0 surface area. Particularly, Korean Patent Application No. 2008-0072286, derived from our previous study about methanol synthesis from synthesis gas, discloses a novel catalyst system, including a Cu—Zn—Al oxide containing CuO, ZnO and Al2O3 in a predetermined ratio, in combination with a cerium-zirconium oxide obtained by a sol-gel process. More particularly, the above patent application relates to a catalyst for synthesizing methanol from synthesis gas and a method for synthesizing the same, wherein the catalyst is capable of inhibiting formation of byproducts and improving selectivity toward methanol as compared to the other catalysts using Cu—Zn—Al alone, thereby improving carbon conversion efficiency and energy efficiency in methanol synthesis.
In addition to the above, according to the existing patent publications about methanol synthesis from synthesis gas, CO2 is reused in a reformer to minimize emission of CO2 produced during POM and a cryogenic separation method is employed in such a manner that H2-rich gas is used for methanol synthesis or as fuel, while CO-rich gas is used in a process for acetic acid preparation, thereby improving the energy efficiency of the overall process (U.S. Pat. No. 7,067,558). Meanwhile, U.S. Pat. No. 6,100,303 discloses a process for improving energy efficiency and CO2 availability, wherein two different types of reformers, i.e., a combustion type reformer and a heat exchanger type reformer, are used for steam reforming of natural gas, and purge gas in the unreacted gases is used as a raw material for reforming/methanol synthesis and as fuel of the reformers in order to reduce the cost required for constructing the processing system and to improve energy efficiency. However, the process of the U.S. Pat. No. 6,100,303 is different from the present disclosure in terms of the construction of the overall process and recycling process. Further, U.S. Pat. No. 6,218,439 discloses a method for utilization of CO2 generated during the reaction based on steam reforming alone, wherein CO2 emission is minimized by refeeding CO2 into reforming or into methanol synthesis after the separation of products. However, the method of the U.S. Pat. No. 6,218,439 patent uses a catalyst different from the catalyst system disclosed herein and is differentiated from the present disclosure in terms of the construction of the overall process and recycling process.